A need exists for threaded fasteners that can withstand vibration cycling with minimum loss of clamping force, i.e., axial load. It is also desirable to provide a fastener having a removal torque that matches or exceeds an installation torque. Such a fastener reduces the effects of tampering.
Known self-locking fasteners that inhibit removal of a tightened nut from a threaded bolt allow the bolt to be rotated as much as 40 degrees before the nut engages so that it no longer rotates. It has been found that a ⅜″ diameter bolt with 16 threads per inch that secures two ½″ plates, so that the length of the bolt under a clamp load is 1″, loses two-thirds of its load on the plates when rotated about 12 degrees in an untightening direction. With the known self-locking fasteners, the bolt and nut may still be retaining the plates after the nut has been rotated 40 degrees in the untightening direction; however, the bolt has lost its load and no longer retains the plates tightly.
Other self-locking fastener assemblies use teeth that engage one another to limit rotational movement of the nut with respect to the bolt. The bolt engages a locking ring that has a plurality of teeth formed on a face that is normal to the longitudinal axis of the bolt. A nut that is threaded onto the bolt also includes a face having a plurality of teeth that engage the teeth of the locking ring. In known assemblies, however, the teeth in the locking ring and the teeth on the bolt are disposed at a positive rake angle in the untightening direction and in the tightening direction. That is, the apex of each tooth follows the point where the base joins the face for each tooth in both the tightening and the untightening rotational direction. Such a configuration allows the teeth of the nut to ramp over the teeth of the locking ring when one attempts to remove the nut. Such movement is undesirable.